Heretofore, there have been known various types of position detection devices which employ a coil (inductance element) as a detection element. Many of such position detection devices include an AC signal source dedicated to energization of the coil, so that the coil is AC-energized by an AC signal generated from the AC signal source being applied to the coil. Also known are proximity sensors which could eliminate the need for a dedicated energizing AC signal source by use of the principle of the LC oscillation circuit, i.e. by incorporating a coil, functioning as a detection element, in a self-oscillation circuit as an inductance element (see, for example, Patent Literature 1). Such self-oscillation type proximity sensors are advantageous in that they can be significantly reduced in size because the need for providing a dedicated energizing AC signal source can be eliminated. However, because the conventionally-known, self-oscillation type proximity sensors are constructed to detect a variation in oscillation frequency corresponding to proximity of an object of detection or detection object, they require a frequency discrimination circuit. Further, although the conventionally-known, self-oscillation type proximity sensors are constructed to be suited for detection of a variation in oscillation frequency, they are not constructed to be capable of detecting a position of an object of detection on the basis of an amplitude level of an oscillation output signal.
If the challenge to reduction in size of the device construction is considered from another perspective, there can be employed an approach of using, as the detection element, a flat coil formed in a swirling shape on a printed circuit board, one example of which is disclosed in Patent Literature 2. In position detection devices using such a flat coil, the number of winding turns in one coil is so small that it is difficult to obtain a sufficient magnetic flux for detection of a detection object. To make up for such an insufficient magnetic flux, the device disclosed in Patent Literature 2 employs a technique of providing the flat coil in multiple layers.